to obtain the desired physical properties correlated to electronic and vibrational states. [1][2][3] In particular, alterations in the intermolecular interactions of organic nonlinear optical materials can result in dramatic changes in the molecular ordering of chromophores, macroscopic nonlinear optical responses, and molecular vibrational and phonon modes, [4][5][6][7] which are important material characteristics in diverse nonlinear optical and THz photonic applications. [8][9][10] The macroscopic nonlinear optical response of organic materials is directly proportional to the first-order hyperpolarizability β of chromophores and their alignments. [8] In many nonlinear optical organic crystals, strong intermolecular interactions to π-conjugated chromophores can adversely affect the resulting macroscopic optical nonlinearity. In well-known and commercially available pyridinium-based crystals, [8,11,12] the first-order hyperpolarizability of chromophores in the crystalline state (β crystal ) is considerably less than in solution (β solution ); e.g., in stilbazolium and merocyanine crystals, β crystal is only about 20% and 5%, respectively, of the corresponding β solution . [6] In a more precise analysis of intermolecular interactions of stilbazoliumbased crystals, reducing intermolecular interactions (edge-toface π-π interactions and hydrogen bonds (H··· − O-S) between A new approach for the molecular design of highly efficient nonlinear optical organic crystals is proposed by introducing substituents that form σ-holes on both nonlinear optical cationic chromophores and aromatic anions. Introducing chlorinated substituents, in which a relatively positive σ-hole and a negative belt coexist, provides selective reduction capability of specific π-π intermolecular interactions and simultaneous multiple secondary bonding capabilities. This leads to a crystalline state with enhanced first-order hyperpolarizability β crystal of chromophores that favors parallel chromophore alignment and suppression of molecular vibrations, which are optimal characteristics for electro-optic and nonlinear optical applications, including efficient THz wave generation. Compared to benchmark nonhalogenated and fluorinated analogous crystals with state-of-the-art macroscopic optical nonlinearity, σ-hole containing chloro-quinolinium crystals exhibit up to two times higher macroscopic nonlinear optical response and remarkably different crystal characteristics. As a result, a 0.16 mm thick chloroquinolinium crystal exhibits ≈22 times higher optical-to-THz conversion efficiency than the widely used 1.0 mm thick ZnTe inorganic crystal. Moreover, chloro-quinolinium crystals exhibit very broad THz spectra, up to 8 THz with significantly different THz spectral shape compared to benchmark organic crystals, which is attributed to different phase matching between optical and THz frequencies and molecular vibration motions.
New organic THz generators are designed herein by molecular engineering of the refractive index, phonon mode, and spatial asymmetry. These benzothiazolium crystals simultaneously satisfy the crucial requirements for efficient THz wave generation, including having nonlinear optical chromophores with parallel alignment that provide large optical nonlinearity; good phase matching for enhancing the THz generation efficiency in the near-infrared region; strong intermolecular interactions that provide restraining THz self-absorption; high solubility that promotes good crystal growth ability; and a plate-like crystal morphology with excellent optical quality. Consequently, the as-grown benzothiazolium crystals exhibit excellent characteristics for THz wave generation, particularly at near-infrared pump wavelengths around 1100 nm, which is very promising given the availability of femtosecond laser sources at this wavelength, where current conventional THz generators deliver relatively low optical-to-THz conversion efficiencies. Compared to a 1.0-mm-thick ZnTe crystal as an inorganic benchmark, the 0.28-mm-thick benzothiazolium crystal yields a 19 times higher peak-to-peak THz electric field with a broader spectral bandwidth (>6.5 THz) when pumped at 1140 nm. The present work provides a valuable approach toward realizing organic crystals that can be pumped by near-infrared sources for efficient THz wave generation.
of the existing organic and inorganic THz wave generators and detectors are hindered by fundamental limitations in spectral generation and detection as well as by bandwidth limits due to intrinsic phonon and molecular vibrational modes. [2][3][4][5] In nonlinear optical frequency conversion processes, such as optical rectification and difference frequency generation, organic electro-optic crystals are more beneficial for broadband THz wave generation and detection as compared to their inorganic counterparts. [6][7][8][9] This is because organic electro-optic crystals exhibit large macroscopic optical nonlinearity with good phase matching ability between optical and THz frequencies and a relatively low absorption coefficient in high-frequency THz spectral regions.When considering THz wave generation and detection, current state-of-the-art organic electro-optic pyridinium-, quinolinium-, and benzothiazolium-based ionic crystals and nonionic phenolic polyene-based crystals exhibit a macroscopic nonlinear optical coefficient one order of magnitude larger than that of inorganic crystals (e.g., ZnTe and GaP) with an effective hyperpolarizability tensor β iii eff > 100 × 10 −30 esu and/or an electro-optic coefficient >50 pm V −1 . [6,[10][11][12][13][14] In addition, benchmark organic electro-optic crystals exhibit much lower absorption coefficients in the high THz frequency region when compared to inorganic crystals. While the upper New organic electro-optic crystals containing orthogonally oriented electronwithdrawing groups are developed for efficient and gap-free THz wave generation with a very flat broadband spectral shape. These crystals consist of 2-(4-hydroxystyryl)-1-methylquinolinium (OHQ) cationic chromophores and nonplanar 4-(trifluoromethoxy)benzenesulfonate (TFO) anions with orthogonally oriented highly electronegative trifluoromethoxy groups capable of strong hydrogen bonds. OHQ-TFO crystals exhibit enhanced macroscopic optical nonlinearity with a second harmonic generation efficiency 2.3 times greater than that of benchmark OHQ-based crystals with conventional planar anions. This enhancement is attributed to reduced edge-to-face π-π interactions between cations and anions due to the orthogonal orientation and electron-withdrawing characteristics of trifluoromethoxy groups. Moreover, OHQ-TFO crystals exhibit excellent THz wave characteristics generated by optical rectification; 0.52 mm thick OHQ-TFO crystal generate a peak-to-peak THz electric field 15 times higher than that of inorganic standard 1.0 mm thick ZnTe crystal and a broader spectrum with an upper cut-off frequency near 8 THz at pump wavelengths of 1140-1500 nm. Unlike previously reported state-of-the-art organic electro-optic salt crystals with strong phonon absorption in the frequency range of 0.8-3 THz, OHQ-TFO crystals facilitate gap-free broadband THz wave generation without strong absorption modulations due to the strong hydrogen bond ability of trifluoromethoxy groups.
Fluorinated electro‐optic crystals with state‐of‐the‐art second‐order nonlinear optical response and excellent characteristics for terahertz (THz) wave generation are reported. The fluorinated organic ionic crystals consist of optically highly nonlinear fluorinated HM6FQ (6‐fluoro‐2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium) or HM7FQ (7‐fluoro‐2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium) cations and 4‐methylbenzenesulfonate (T) counter anions. Compared to benchmark electro‐optic crystals based on nonfluorinated HMQ (2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium) cations, introducing fluorine substituent on HM6FQ cations creates additional hydrogen bonds (ArF···HC). Such a molecular engineering leads to an enhanced thermal stability and significant modulations of phonon vibrational modes of crystals in THz frequency region, while excellent π–π stacking and space filling characteristics of HM6FQ cations in crystals lead to state‐of‐the‐art diagonal component of the macroscopic nonlinear optical susceptibility, similar to the case of HMQ cations. HM6FQ‐based crystals exhibit a very high optical‐to‐THz conversion efficiency, comparable to benchmark HMQ‐based crystals. In addition, due to additional hydrogen bonds induced by fluorine substituents, the spectral shape of the generated THz wave is remarkably modified; e.g., the largest spectral gap is near 1.5 and 2.0 THz for HM6FQ‐ and HMQ‐based crystals, respectively. The fluorinated cationic engineering on nonlinear optical crystals having benchmark nonlinear optical response is, as far as is known, reported for the first time.
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